1. Field of the Invention
The present invention relates generally to an apparatus and a method for performing printing. More specifically, the invention relates to a printing apparatus and a printing method for performing gradation or gray scale printing on a printing medium employing at least two kinds of inks of one color with mutually different density.
2. Description of the Related Art
As currently major printing system, there are a thermal transfer type printing system transferring an ink on an ink ribbon onto a printing medium by a thermal energy, an ink-jet printing system performing printing by ejecting a liquid, such as an ink to deposit on the printing medium, and the like.
Among the features of the ink-jet type printing system are capability of high speed printing, low noise generation during printing operation, easiness of color printing with employing a plurality of colors of inks for printing, and capability of desired printing on a plane paper without requiring special fixing process. Furthermore, by employing at least two kinds of inks with different density for one color (e.g. cyan), high gray scale printing can be facilitated.
FIG. 4 shows an example of setting of ink ejection amounts of respective of higher density ink and a lower density ink for a predetermined unit image with respect to a multi-valued input signal level (0 to 255) upon formation of three-value level image employing two kinds of inks of one color and different densities (respectively referred to as higher density ink and lower density ink). In general, when three-value level image is formed utilizing two kinds of inks of different densities, gray scale expression is performed by presence and absence of lower density ink dot within a region A in the drawing. At a predetermined point B, 100% ejection of the lower density ink is performed. In a region C, the lower density ink dot of 100% ejection at the B point is gradually replaced with higher density ink dot to finally perform 100% ejection of higher density ink dot in the input signal level of "255", and thus to perform three-value level gray scale or gradation expression.
The region A where the signal value is low, is the case, such as for forming a hilighted portion in a general photograph image or the like. Granular feeling of the image in the hilighted portion is lowered at lower reflection density ("Optical Density" or "OD" value) of the image formed by one lower density ink droplet. However, the signal region exceeding a point B is a region where a few higher density ink dots are scattered in the lower density ink dots. Therefore, in such region, if OD values of the higher density ink dots and the lower density ink dots are significantly different, granular appearance of the image is increased.
Namely, in the prior art, no consideration has been given in that, upon performed multi-valued printing employing a plurality of kinds of inks of one color with different densities, granular appearance by the lower density ink dots formed by the lower density ink in the low gray scale region, and granular appearance by the higher density ink dots formed by the higher density ink in a transition region where lower density ink dots are changed into the higher density ink dots, are reduced with good balance.